The present invention relates to a method for the preparation of a foamed and cured silicone rubber body, more particularly, to an efficient method for the preparation of a foamed and cured silicone rubber body having a very uniform and dense cellular structure.
As is well known, silicone rubber compositions are cured into various cured silicone rubber bodies by different curing methods depending on the types of the silicone rubber composition and the desired form of the silicone rubber articles. When a continuous-length body of a cured silicone rubber composition is desired, for example, the uncured silicone rubber composition is extrusion-molded into a continuous-length body, such as tubes, sheets and the like, which is then cured or vulcanized by the method of normal-pressure hot-air vulcanization, continuous steaming vulcanization, molten-salt vulcanization or the like. Although these conventional methods are useful when the desired cured silicone rubber body has a solid or non-cellular structure, none of them is applicable to the preparation of a continuous-length body of a cured silicone rubber having a foamed or cellular structure with uniform and dense distribution of fine cells.
Apart from silicone rubbers, the demand for foamed and cured rubber bodies produced, in particular, in a continuous-length form is rapidly growing in recent years in various applications such as gaskets and heat- and sound-insulators in buildings, rubber rollers in office machines and the like. In this regard, an efficient method has been developed for the preparation of a cured and foamed body of various organic rubbers in general in a continuous-length form, in which a curable and foamable composition of a rubber such as ethylene-propylene-diene copolymeric rubbers, polychloroprene rubbers and the like is extrusion-molded into a continuous-length form which is then subjected to irradiation with ultrahigh-frequency electromagnetic waves or microwaves to generate a sufficient quantity of heat in the body of the rubber composition as extruded so that the rubber composition is foamed and cured into a cured and foamed rubber body.
This method of rubber-vulcanization by the ultrahigh-frequency irradiation, referred to as the UHFV method hereinafter, however, is not applicable to the preparation of a cured and foamed silicone rubber body of a continuous length due to the loss index of silicone rubber compositions in general is so small an insufficient quantity of heat is generated. More specifically, the UHFV method is performed by exposing a shaped body of an uncured rubber composition to the field of electromagnetic waves at a frequency of 2450.+-.50 MHz or 915.+-.25 MHz according to the regulation so that the rubber body absorbs the energy of the electromagnetic waves to generate heat by which the crosslinking reaction proceeds in the rubber body to give a cured rubber body. When a rubber body in a microwave-heating oven is irradiated with microwaves, the rate of energy absorption by the rubber body P in watts/m.sup.3 is given by the equation: EQU P=(5/9)f.multidot.E.sup.2 .multidot..epsilon..multidot.tan .delta..times.10.sup.10,
in which
f is the frequency of the microwaves in hertz (Hz); PA1 E is the high-frequency electric field in volts/meter; PA1 .epsilon. is the relative dielectric constant of the rubber; and PA1 tan .epsilon. is the dielectric loss factor of the rubber. PA1 (A) compounding, into a uniform composition, PA1 (B) irradiating the composition prepared in step (A) above with microwaves at a frequency in the range from 900 to 5000 MHz.
The value of .epsilon..multidot.tan .delta. is usually called the loss index of the material and it is generally understood that the UHFV method is applicable only to a rubber composition having the loss index of at least 0.08 or, preferably, at least 0.2. Silicone rubber compositions in general have a much smaller loss index of only about 0.03 at a frequency of 3000 MHz than most of other organic rubber compositions. This is the principal reason for the general understanding that the UHFV method is hardly applicable to silicone rubber compositions.
In view of the above mentioned problem relative to the applicability of the UHFV method to silicone rubber compositions, a proposal has been made in Japanese Patent Kokai 52-37963 with an object to improve the efficiency of the UHFV method for silicone rubber compositions, according to which at least 5% by moles fraction of the organic groups bonded to the silicone atoms in the organopolysiloxane as the major constituent of the silicone rubber composition are selected from the class consisting of aryl groups, chlorinated aliphatic hydrocarbon groups, fluorinated aliphatic hydrocarbon groups, hydrocarbon groups having at least one mercapto group bonded to the carbon atom, hydrocarbon groups having at least one methylol group and alkoxyalkyl groups. A problem in the use of such a modified organopolysiloxane as the constituent of a silicone rubber composition is that the cured silicone rubber articles prepared from the composition are not always quite satisfactory from the practical standpoint in respect of the heat resistance, weatherability, electric properties and surface properties.